Process for applying a polymeric film to a steel substrate

ABSTRACT

A METHOD OF APPLYING A FILM OF LOWER POLYOLEFIN OR FIM OF A COPOLYMER OF A MAJOR AMOUNT OF A LOWER OLEFIN WITH A MINOR AMOUNT OF A COPOLYMERIZABLE MONOMER HAVING POLAR FUNCTIONAL GROUPS TO A STEEL SUBSTRATE WHICH INVOLVES GENERATION OF AN INTERMEDIATE LAYER OF CHROMIUM/CHROMIUM OXIDE ON THE SUBSTRATE AND THEN APPLICATION OF THE   POLYOLEFIN FILM DURING A PRE-DETERMINED HEATING CYCLE IS DESCRIBED. THE METHOD PERMITS HIGH LINE SPEEDS IN A CONTINUOUS PROCESS. THE PRODUCT EXHIBITS EXCELLENT RESISTANCE TO CORROSION AND DELAMINATION.

BEST AVAILABLE CQPY July 25, 1972 R ADDINALL EI'AL 3,679,513

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OTOKAR MU'DROCH BY modfl' I ATTORNEYS BEST AVAILABLE COPY July 25, 1972 R. ADDINALL EI'AL 3,679,513

PROCESS FOR APPLYING A POLYMERIC FILM TO A STEEL SUBSTRATE Filed May 6, 1970 I 2 Sheets-Sheet 2 OLYETHYLENE COATING FIG 2 LIJ 9 6 1 55 2 d g INVENTORS w 0 U LRAMON L.ADDINALL WILLIAM T. LEWIS OTOKAR MUDROCH BY same-A? a-QQW ATTORNEYS 3,679,513 Patented July 25, 1972 BEST AVAEABLE COPY 3,679,513 PROCESS FOR APPLYING A POLYMERIC FILM TO A STEEL SUBSTRATE Ramon L. Addinall, Stoney Creek, Ontario, William T. Lewis, Burlington, Ontario, and Otolrar Mudroch, Hamilton, Ontario, Canada, asaignors to The Steel Company of Canada Limited, Hamilton, Ontario, Canada Filed May 6, 1970 Ser. No. 35,102 Int. Cl. (509i 5/06 US. Cl. 156-309 9 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION (1) Field of invention This invention relates to a method for producing a corrosion resistant coating on a metal substrate. In particular, it relates to the application of a polymeric organic film to a steel substrate on which a chromium/chromium oxide film was previously generated.

(2) Description of prior art While traditionally the plastics and the metals industries have considered themselves to be in competition with one another in the marketing of their products it has been recognized by both factions that metals and plastics have essentially complementary properties. A metal with the corrosion resistance and flexibility of a plastic or a plastic with the rigidity and strength of metal have both been sought. A compromise solution to these problems is a product comprising a metal substrate which is coated with a plastic. The least expensive product fitting this description would be a substrate of the least expensive metal, steel, coated with a film of the least expensive plastic, polyethylene, but this idealised product has been found to be very diflicult to prepare. Furthermore, because polyethylene is at least to a certain extent water permeable, such a product would not be completely corrosion resistant.

It is also known to apply a high density layer of polyethylene directly to steel and then apply a low or medium density polyethylene layer to the high density layer. However, the steel must be heated to a sufiiciently high temperature for a sufficiently long period of time so that the high density polyethylene is completely melted and so that complete melting occurs in both layers at the junction therebetween. It does not appear that such a method can be carried out as other than a batch process when polyethylene sheeting is employed.

As mentioned above, a polyethylene film is not completely impermeable to water; thin films of low molecular weight polyethylene are most permeable than thick films of high molecular weight. Because of this general property it is doubtful that even the double layer coating of both types as described would be completely impermeable.

One branch of commerce with a direct interest in the development of a metal substrate having an organic coating is the can manufacturing industry. Recently it has been found that a chromium/chromium oxide composite coating is a suitable replacement for tin on steel container stock. There are two preferred methods of applying such a coating to steel: A so-called two-step process introduced by the National Steel Corporation of Weirton, West Va., and a one-step process as described in Canadian Patent 808,630 which issued on Mar. 18, 1969 to Toyo Kohan Co. Ltd. While such a coating exhibits by itself commercially desirable properties, it has been found that applying a further coating of an epoxy-phenolic resin produces a product with a more pleasing finish and which further protects the coating from abrasion. Unfortunately this product is attained with the additional costs of the resin itself and of its application. .As a result, epoxyphenolic systems are recognized as the highest performance/highest cost resin systems available, the standard of excellence for the industry.

It can be seen that there is a need for a less expensive replacement for epoxy-phenolic resin as a coating for chromium/ chromium oxide coated steel. Polyethylene, being the least expensive plastic, is the natural choice as the most desirable substitute for the epoxy-phenolic resin. Polyethylene is a substantially inert substance which is a useful property in a lining for a beverage can but unfortunately this same chemical inertness makes it difiicult to satisfactorily bond it to any substrate. For example, if a film of polyethylene is applied to a metal substrate which is subsequently heated, the film blisters and shrinks and can be readily peeled from the substrate.

While some bonding processes have succeeded in obtaining a satisfactory bonding of polyethylene to a metal substrate such processes have not been a commercial success because of the expense involved. None of these have involved the simple direct application of a single film of polyethylene to a metallic substrate which can be economically carried out on a commercial scale in a continuous line process.

SUMMARY OF INVENTION We have now found that a film of polyolefin can be applied directly to a chromium/chromium oxide coating after the coating has been heated and the polyolefin film is strongly adhered to the coating. A useful heating cycle was empirically determined whereby lower polyolefins and copolymers of a major amount of a lower olefin with a minor amount of a copolymerizable monomer having polar function groups could be coated onto a chromium/ chromium oxide surface of a steel substrate.

An object of this invention is to provide a process whereby a lower polyolefin or a copolymer of a major amount of a lower olefin with a minor amount of a copolymerizable monomer having polar functional groups can be coated onto steel. This is accomplished through application of a film of polymer to a chromium/chromium oxide coated steel substrate during a controlled heating cycle. The preferred polymeric film of the invention is a polyethylene film.

The product of this invention is a steel substrate coated with chromium/ chromium oxide to which is adhered a film of a lower polyolefin or a copolymer of a major amount of a lower olefin with a minor amount of a copolymerizable monomer. having polar functional groups. The said product exhibits corrosion resistance and resistance to delamination and peeling comparable to that exhibited by an I identical substrate coated with an epoxy-phenolic resin, but can be produced at a lower cost than the latter.

tions such as in beverage cans, automobile gas tanks and in laminating sheets of steel together. A sand filled polyethylene coated substrate can be used for roofing or other building materials. The product can be stretched and molded within practical limits without appreciable loss of corrosion resistance or lessening of adhesion of film to substrate.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of an apparatus for carrying out a preferred embodiment of the process of the invention. 7

FIG. 2 is a section through the coated product illustrat- 1 ing diagrammatically its composition. The drawing is not doneto scale.

FIG. 3 is a plot of a typical heating profile employed in carrying out the process. Temperature is plotted on the ordinate and time on the abscissa.

- DESCRIPTION OF THE PREFERRED EMBODIMENTS The heating cycle shown in FIG. 3 is carried out on the apparatus of FIG. 1 in one embodiment of this invention. The steel. strip is first coated (in this instance on both sides) with a chromium/chromium oxide coating in a step not constituting a part of this invention and hence not shown.

FIG. 2 shows a section made through the product of this invention. Using standard plating techniques of the tin free steel art such as'the two-step process of National Steel Corporation or the one-step process described in Canadian Pat. 808,630 mentioned above, discreet layers of chromium metal and chromium oxide are formed. The thickness of these layers may be controlled by varying plating bath parameters as is well known in theart.

The layers of metal and of oxide are too thin to be measured by conventional techniques and so thickness is determined by the value in milligrams of chromium that can be leached from one square foot of surface of the I substrate; the oxide by leaching with a base, the metal by leaching with an. acid. Preferably the layers are of approximately equal thickness, the chromium values for the composite coating varying between above about 0.1 mg./ ft. to about mg./ft.=. Below 0.1 mg./ft. corrosion resistance is unsatisfactory while above about 15 mg./ft. possible cohesive failure within the coating will begin to be aconcern. The preferred range of values for each layer isaboveabout 1 mg./ft. to about 4 mg./ft..

Chromium-in-oxide is the accepted trade method for reporting the thickness of the oxide layer of thecoating. I

The chromium oxide consists essentially of Cr+ plus minor amounts of Cr". To quote the'weight or thickness of oxide would bemeaningless because the degree of hydration is indefinite and variable. Therefore the chromiumin-oxide value is the total non-metallic chromiumleached from the substrate. In this description chromium metal value will refer to the total metallic chromium leached 7 from the substrate.

In the example described below the polyolefin film used is polyethylene, but it is to be understood that the invention is not restricted tothis one type of polyolefin film.

ably kept well below polyethylene softening temperature and are preferably coated with polytetrafiuoroethylene to eliminate the possibility of the polyethylene tacking to them. A cooling BC occurs in the heating profile shown, but it is possible to maintain the temperature of the substrate at B by, say, passing an electric currentthrough it without any deleterious effect on the process. 7

If the upper polyethylene payoff reel and the upper nip roll are removed and the roll over which the coated steel strip' passes before entering the preheat furnace lowered so that a pressure isplaced on the lower polyethylene film from below by the nip roll and from above by the coated steel the film will still be intimately adhered to the coated steel strip.

. Reheating of the coated substrate is represented by CD of the curve in FIG. 3 as the coated substrate passes through the induction coil and attains its maximum temperature at D. This temperature is maintained, as represented by curve DE, in the post heating zone. The residence time in portion DE is widely variable and depends largely on the thickness or the type of the polyethylene film. We have found residence times from 5 to 240 seconds to be satisfactory. In order to ensure the fastest possible line speeds it is preferable to keep thisresidence time as short as possible.

The maximum temperature attained must be below the degradation point of the polyethylene, indicated by the numeral 2 on the ordinate'of the graph in FIG. 3, but the higher the temperature the shorter is the post heating time. The degradation point is the temperature at which the polyethylene begins to smoke and discolour and this temperaturevaries among types of polyethylenes. 7

Rapid cooling occurs at EF in the curve. Forced air is used to lower the temperature of the coated substrate to below the softening point; the surface of the film is not roughened by this treatment. Final rapid cooling occurs under the water spray. While it is quite possible to allow the product to .coolin still air, maximum line speeds are promoted by the quickest possible cooling and no deterioration of product was observed using this method.

It is difficult to give a numerical value to points 1 and I and 2 of FIG. 3 as these vary between types of polyethylene. Polyethylene films with the same densities and melt indices but made by different manufacturers have been found to have different values and it is 'believed that the values are determined by the distribution of molecular weights within the film. In the circumstances, attaching numerical values to points 1 and 2 is not very meaningful, but it has been found that, in general, point 1 should not be below C. and point 2 should not be above 300 C. a A number of polyethylene films have been used with success although the process is preferably carried out using 7 low or medium density polyethylene. Ithas been found that long residence times in the post heating zone are required when high density polyethylene is used. The thickness of the film may be varied within practical limits and the man skilled in the art may easily determine just what Useful polymeric films which have been coated onto prepared substrates are listed below:

Film thickness, Code name Supplier inches Remarks Surlyn 1 Du Pont 0. 002 Ionomeric. K M C.I.L 0.002 Melt index 0.5. C.I.L 0. 003 Melt index 1.6. K 708 Z 0.1.L 0.003 Melt index 2.0.

l Surlyn is a registered trademark of Du Pont for a product described as an ionized olefin copolymer having interchain forces developed between ionized carboxyl groups pendant from an a-olefin chain and common metallic ions.

These are code numbers for three types of polyethylene products supplied by C.I.L. They were chosen arbitrarily and while they are typical of polyethylene films employed, the process of this invention is by no means restricted to the use of these films. It is to be expected that films of both homoand copolymers of lower olefins can also be employed and the expression lower polyolefin used herein is meant to encompass such films.

The special case of Surlyn deserves some comment. It was found that this film was :firmly coated onto the prepared substrate after only a very short residence time in the post heating zone and that the coated product exhibited corrosion resistance comparable to that of the polyethylene coated substrate. One disadvantageous property of the product is that the film clouds when the product is heated to temperatures above 120 F. although such clouding does not appear to affect the corrosion resistance of the product.

It was found in a series of tests described below that products using Surlyn exhibit corrosion resistance comparable to that of products using polyethylene while at the same time exhibiting resistance to peeling and to delamination superior to that of the polyethylene products.

These observations lead us to predict that copolymers of a major portion of a lower olefin and a minor amount many other practical ways. One additional test shows the resistance of the product to the action of gasoline.

Immersion tests TABLE 1 Time to failure, days Present inven- Test control coating Can manution coating iacturer E,- P.E.- Surstandard P.E.- Sur- 0orrosive agent B.P. B.P. B.P. E.-0.C. 0.0. 0.0.

3%NaCl 2 1 1 10 10 10 3% HaPOi 6 1 1 10 10 10 136% NaCl plus Beer 30 30 30 30 30 30 1 Failure was undercutting rather than delamination. 2 0.5 mm. after 30. 1 1 mm. after 30.

[A Cleveland condensing humidity cabinet, Q.C.T. cycle was used to test samples X-hatched and edge masked as above. 12 cycles are considered to be equivalent to ap proximately 1.5 years of outdoor exposure.

TABLE 2 120 F., 100% (relative humidity) percent rusting Present inven- Can manution coating Test control coating iacturer standard P.E Sur- Duration E.-B.P. P.E.-B.P. Sun-B.P. E.-C.0. 0.0. 0.0.

Cycle:

1 0%, 1.5 mm. 5%, delamination- 5%, delamination- 0 0 0 undercut. 3 5%, 2.5 mm. 0 0 0 undercut. 7 10%, delamination n 0 0 0 l2 0 0 0 of a copolymerizable monomer having a polar functional group may be more strongly bonded to the substrate than polyethylene. While it may be that such films are more expensive than polyethylene, short residence times in the substrates coated with chromium/ chromium oxide to which polyethylene has been bonded in the manner described has corrosion resistance and resistance to delamination and peeling comparable to lacquered steel now commonly used in the can making industry. While the tests evaluate the product for use in this specific application this was done simply as a matter of convenience and it will be appreciated that these properties can be utilized in It can be see from Tables 1 and 2 that the chromium/ chromium oxide composite coated steel to which is applied olefin polymers equals the can manufacturing industry standard (E-C.C.) in terms of corrosion resistance.

Drawability It was found that the porosity of P.E.-C.C. was not changed by drawing within the limits of what the metal will stand.

Physical adhesion A scrape adhesion test, ASTM D 2197-67T, showed comparable values for P.E.-C.C. and for E.-C.C.

A peel adhesion test was carried out on P.E.-C.C. It showed consistant peel values greater than 40 lbs./% strip. The standard minimum acceptable peel for lacquered can manufacturing stock is 40 lbs., but this arbitrary figure already represents a large safety factor and it is more meaningful to point out that peel values of less than 10 lbs./% strip indicate poor adhesive strength. Examination of peeled P.S.-C.C. samples indicated that cohesive rather than adhesive failure had, in general, occurred.

In summary, the above tests show that the product of this invention has properties which meet the standards set by the can manufacturing industry and that it is a suitable replacement for the lacquer-coated canning stock now widely used.

Gasoline immersion Samples of Sur-C.C., P.E.-C.C. and terneplate (traditional material for gasoline tanks) were exposed to gasoline at various temperatures over various periods of time in a Soxhlet extractor which produces a flushing action on the sample. Additionally static immersion tests at room temperature were performed. Low octane rating commercial grade gasoline containing one percent distilled water was employed to simulate a fuel with high moisture content.

The terneplate did not show significant change in exposure up to boiling temperature and over a long period of time. Both Sur-C.C. and P.E.-C.C. exhibited no loss of adhesion exposed at temperatures below 120 F. and no change in colour due to corrosion. Surlyn showed a faint white milkiness at 130 F. One minor disadvantage pipes are used commercially, this does not appear to be a practical limitation.

The use of our product as gasoline tanks olfers several advantages.'The P.E.-C.C. sheet can :be bent and drawn to a desired shape without loss of corrosion resistance. Two halves of a tank can be pressed together and an excellent seal attained by melting and then cooling the adjacent polyethylene film at the joint.

What we claim is:

1. A process for coating a steel substrate with a polymeric film which comprises:

(a) generating a chromium/chromium oxide composite coating'on the steel substrate, the composite coat ing comprising -(1) adjacent the steel substrate a chromium metal layer having a chromium metal value between above mg./ft. to about 15 'mg./ft.', and

(2) adjacent the chromium metal layer a chromium oxide layer having a chromium-in-oxide value between above 0 mgjft. to about 15 mg./ ft.', said composite coating having a total chromium value above about 0.1 mg./ft. to about 15 mgJftF,

(b) heating the so-called substrate to a predetermined temperature at least as high as the softening point but below the degradation point of the polymer,

(c) contacting and adhering the polymeric film with the heated so-called coated substrate at the nip of a pair of rollers, said rollers being maintained at a temperature substantially below the softening point of 8 the polymeric film, the polymeric film being a film of a lower polyolefin or a copolymer of a major amount of a lower olefin with a minor amount of a copolymerizable monomer having a polar functional (d) subjecting the laminate from (c) to a temperature above the softening point but below the degradation point of the polymeric film until the film achieves intimate contact with the microstructure of the surface, and

(e) cooling the coated substrate.

2. Process according to claim 1 wherein said polymeric film is a polyethylene film.

3. Process according to claim 1 wherein said polymeric film is a film of an ionized olefin copolymer in which interchain forces are developed between ionized carboxyl groups pendant from an a-olefin chain and common metallic groups.

4. Process according to claim 1 wherein said chromium/chromium oxide coated substrate is heated to at least C. before contacting it with the polymeric film.

5. Process according to claim '1 wherein said polymeric film is contacted and adheredwith said heated substratev at the nip of a pair of rollers, said rollers being maintained at a temperature substantially below the softening point of the polymeric film.

6. Process according to claim continuously.

7. Process according to claim 1 wherein any roller in contact with said polymeric film is coated with polytetrafluoroethylene.

1 whenever carried out 8. Process according to claim -1 wherein said laminate from (c) is subjected to a temperature up to about 300 C.

9. Process according to claim 1 wherein said chromium metal value and said chromium-in-oxide value are each above 1 to about 4 mg./ft..

'10. Process according to claim 1 wherein said laminate from (c) is heated to atemperature up to 300 C. for from 5 to 240 seconds.

References Cited UNITED STATES PATENTS 

